1. Field of the Invention
The present invention relates to reception of signals from known sources, and more specifically to predicting the content of signals received from known sources.
2. Description of Related Art
Ease and accuracy of position and time determination has increased significantly since the development of the well-know Navigation Satellite Timing and Ranging (NAVSTAR) Global Positioning Satellite (GPS) System. The NAVSTAR GPS system is described in Global Positioning System Standard Positioning Service Signal Specification, 2nd edition, Jun. 2, 1995, United States Coast Guard Navigation Center, Alexandria, Va. Another such system is the Global Navigation Satellite System (GLONASS) GPS system maintained by the Russian Republic. GPS receivers are currently available for use in aircraft, ships, and ground vehicles and for hand carrying by individuals.
The NAVSTAR GPS system provides for thirty-two satellites or xe2x80x98space vehiclesxe2x80x99 (SVs) that orbit the earth in six orbital planes (four satellites, plus spares, in each plane). Each SV orbit repeats almost the same ground track each day as the earth turns beneath the SVs of the system. The orbital planes are equally spaced and inclined with respect to the equatorial plane, thus ensuring that a line-of-sight path exists to at least five SVs from any (unobstructed) point on the earth.
Ground-based monitor stations measure signals from the SVs and incorporate these measurements into orbital models for each satellite. Orbital data and SV clock corrections are computed for each satellite from these models and uploaded to each SV. The SV then transmits information relating to its position at a data rate of 50 bits per second, via BPSK modulating a direct sequence spread spectrum signal having a chip rate of 1.023 MHz that is modulated onto an RF carrier, with each SV using a different spreading code (also called a Gold code or a coarse acquisition or C/A code). Hereinafter the information carried on the signal transmitted by a SV is called xe2x80x98navigation data.xe2x80x99
A GPS receiver calculates its position by combining navigation data that indicates the position of the SVs with the delay or phase of the signal received from the SVs (which indicates the distance between the receiver and the SVs). Because of inaccuracies in the receiver""s timebase oscillator, signals from at least four SVs are required to resolve a position in three dimensions, although signals from additional SVs (if available) may be used to provide better accuracy.
It is desirable to augment certain wireless systems for mobile communications by adding the capability to locate the position of a particular mobile unit. One reason is a regulation promulgated by the Federal Communications Commission (FCC) (Docket No. 94-102, Third Report and Order adopted Sep. 15, 1999, released Oct. 6, 1999) which requires all cellular carriers in the United States to be able to locate the position of a cellular telephone making an emergency 911 call within 50 meters for 67 percent of calls and within 150 meters for 95 percent of calls by October 2001. Other uses for position location capability in wireless communications systems include value-added consumer features such as navigation and vehicle fleet management support.
One possible approach to supporting position location in a wireless communications system is to add GPS location capabilities to the mobile units. However, GPS receivers generally require unobstructed, strong signals, which may not be available to a mobile unit. GPS signal detection under unfavorable SNR conditions (for example, inside a building or vehicle where no direct line of sight can be established from the receiver to at least four SVs) is a continuing problem.
In order to detect the GPS signal at the receiver, a matched filter may be used to generate the spreading code and apply it to the received signal in a search for a correlation peak. This method is called coherent integration. Short coherent integration denotes integration over a period of less than one data bit (in the case of a GPS signal, less than 20 milliseconds), while long coherent integration denotes integration over a period of more than one data bit. It may be desirable to apply long coherent integration, as a longer integration period may allow for a higher processing gain.
One drawback to using long coherent integration on a signal such as a GPS signal is that the data modulated onto it may integrate to produce an output of low or zero magnitude. In other words, if each binary data symbol that is modulated onto the signal (expressed herein as xe2x80x980xe2x80x99 or xe2x80x981xe2x80x99) appears as frequently as the other binary data symbol during the integration period, then the output of the matched filter over the integration period will sum to zero and no signal will be detected. Even if the data symbols sum to a nonzero value, the resulting receiver performance will be severely reduced in most cases.
U.S. Pat. No. 6,118,977, issued to Vannucci, discloses a method where the known GPS navigation data is used in a mobile receiver to generate a local replica of the transmitted signal, which includes representation of the data modulation on the signal. This allows the mobile receiver to correlate the received signal utilizing long coherent integration period without experiencing the output degradation explained above. The method of Pat. No. ""977 requires carrying out the processing steps described subsequently in this paragraph. The mobile receiver stores processed data samples of the received signal to a FIFO memory for later off-line processing. An auxiliary system, which has unobstructed view of all SVs above the horizon, receives GPS signals at the same time with the mobile receiver and demodulates the transmitted navigation data bits. The auxiliary system conveys the demodulated navigation data bits to the mobile receiver. The mobile receiver off-line processes the stored data samples, utilizing the navigation data bits conveyed to it by the auxiliary system.
Unfortunately, the memory capacity needed for storing processed data samples in the mobile receiver makes the method of Pat. No. ""977 impractical. The required memory capacity is dictated by the data sampling rate, the coherent integration length, the number of SVs and the number of code phase and Doppler hypotheses for which signal search is attempted simultaneously. Note that a typical mobile receiver cannot detect signals from both the SVs and the auxiliary unit at the same time, thus, the required memory capacity cannot be reduced by cyclic operation (i.e. when the same memory cell is reused for storing different segments of the received signal while maintaining coherent integration).
Thus, there is a need for a method, apparatus and system that would allow a receiver to perform long coherent integration on signal receivers without the undue burden of having to incorporate large-scale memory devices in those receivers.